Functional oriented metrics for software ranging

I have a problem. I need to evaluate the size of software product. I know, that there are LOC-oriented metrics (Lines Of Code) and functional oriented (FP - Functional Points) metrics for this aim. There is a lot of theoreical materials for this topic, but it seems very difficult to use this theory in practice. It's interesting to know, did some one use this method? I will be grateful if you help me with your experience..